(Parts of the following section may not be prior art.)
Optical fiber lasers are available that produce optical pulses with high pulse energy, good beam quality and excellent optical characteristics. Several applications for these optical pulse lasers exist, ranging from time-resolved near-field scanning optical microscopy (NSOM) pump-probe experiments for understanding ultrafast electronic processes in materials (see S. Smith, N. C. R. Holme, B. Orr, R. Kopelman and T. B. Norris, “Ultrafast measurement in GaAs thin films using NSOM,” Ultramicroscopy, vol. 71, pp. 213-223, 1998); for two-photon fluorescence of dyes (see A. Lago, A. T. Obeidat, A. E. Kaplan, J. B. Khurgin, P. L. Shkilnikov and M. D. Stern, “Two-photon-induced fluorescence of biological markers based on optical fiber,” Optics Letters, vol. 20, pp. 2054-2056, 1995); for studying biological processed in living tissues (see G. Alexandrakis, E. B. Brown, R. T. Tong, T. D. McKee, R. B. Campbell, Y. Boucher, and R. K. Jain, “Two-photon fluorescence correlation microscopy reveals the two-photon natures of transport in tumors,” Nature Medicine, vol. 10, pp. 203-207, 2004). The last application has potential impact on the prospects for non-invasive cancer detection schemes where the delivery fiber is an endoscope (see E. B. Brown, Y. Boucher, S. Nasser, R. K. Jain, “Measurement of macromolecular diffusion coefficients in human tumors,” Microvascular Research, vol. 67, pp. 231-236, 2004).
For the case of studying live tissues, the fs pulse acts as the pump beam that excites fluorescence mediated by a 2-photon process. Since multi-photo processes are by nature inefficient, high peak powers are needed. However, this cannot be achieved by increasing the average power of the source, because high average power will cause tissue damage. Hence, such applications typically require pulses of the duration of roughly 100 fs, with pulse energies as high as 1 nJ, while the average power is maintained at roughly 100 mW or lower. A commonly used laser source for such schemes is a mode-locked Ti:Sapphire laser that can output very high peak powers with repitition rates of ˜80 MHz.
The delivery fiber desirably propagates the high power, short pulses through a (typically) 1-2 meter-long fiber, and provides an output that is close in characteristics to the laser output. However, there are two physical constraints that affect the output from the delivery fiber. The dispersion of the fiber, due to material as well as waveguide dispersion, leads to pulse broadening that transforms the 100-fs pulse at the input of the fiber into 10-20 ps long pulse at the fiber-output. In addition, since the peak power levels are so high, nonlinear phase shifts due self-phase modulation (SPM) lead to a narrowing of the spectral width of the pulse, further broadening the pulse. The dispersion effect is linear, and thus may be compensated by a bulk linear chirp element between the Ti:Sapphire output and fiber input. The linear chirp element may be a bulk grating or prism pair used to stretch or compress pulses. Such elements are capable of providing arbitrary amounts of positive or negative dispersion. For this application, they may be adjusted to provide dispersion that is equal in magnitude, but opposite in sign to that of the specified length of the fiber endoscope/delivery medium.
Accordingly, while the dispersion problem may be addressed with some effectiveness, the nonlinear SPM effect is non-recoverable. Hence, a majority of fiber delivery schemes work with special fibers or complicated phase engineering of pulses to counteract the SPM effect.
In a high performance system the delivery fiber should also be a single mode fiber with low loss. Propagation in multiple modes degrades the ability to tightly focus the output from the fiber. A tightly focused output enables concentrating the high peak power pulse on a small region, thus enabling efficient 2-photon fluorescence, as well as ensuring high resolution for microscopy applications. Propagation in multiple modes also spreads the pulses due to modal dispersion, which lowers the peak power and reduces the efficiency of nonlinear measurement techniques.
Low bend losses are desirable in applications, such as endoscopes, where the fiber, even though short in overall length, may still undergo multiple bends. Connection losses include the concatenation of a collimating element at the fiber output. This element will focus the output to a tight spot. Candidates for the collimating element are fiber-GRIN lenses, or thin-film-based diffractive optic elements such as mode transformers, or other beam shaping elements. Normally, such miniature beam shaping elements can be epoxy-bonded to the tip of the fiber, but considering that very high peak powers will emanate from the fiber, it is desirable that a mode transforming element such a long-period grating or GRIN lens be used, since they are fiber-based, and can be easily fusion spliced to the output of the fiber, with low loss and high power handling capability.
The baseline candidate for the delivery fiber is a standard fiber (doped core, and silica cladding) which is single-moded at the desired wavelength of operation. A typical desired operating wavelength is ˜800 nm (for Ti:Sapphire lasers), and the effective area (Aeff) of a standard single mode fiber (SMF) at this wavelength is <25 μm2. This fiber satisfies all the above criteria, but at pulse energies >0.1 nJ, SPM severely distorts the pulse. The pulse width rapidly expands past 250 fs (desired pulse widths are <200 fs).
A variety of solutions to the SPM problem have been proposed. Among these are using a multimoded fiber, but forcing signal propagation in the fundamental mode to enable signal propagation in a large Aeff, thus decreasing SPM. See F. Helmchen, D. W. Tank and W. Denk, “Enhanced two-photon excitation through optical fiber by single-mode propagation in a large core,” Applied Optics, vol. 41, pp. 2930-2934, 2002. However, pulse widths obtained with this method are still undesirably large, especially for two-photon applications.
A variation of the above solution is to use a large core, multimoded microstructure fiber. See D. Ouzounov, K. Moll, M. Foster, W. Zipfel, W. W. Webb and A. L. Gaeta, “Delivery of nanojoule femtosecond pulses through large-core microstructured fibers,” Optics Letters, vol. 27, pp. 1513-1515, 2002. Microstructured fibers are guided by a photonic crystal of air holes running through the glass fiber, and this mitigates mode coupling problems. However, it appears that coupling effectively only into the fundamental mode in microstructured fibers is a problem, and significant power is lost to higher order modes. This causes unwanted modal noise in the system. In addition, microstructured fibers have poor geometric control compared to standard doped fibers, and a potential drawback is geometric ovalities that would cause large polarization mode dispersion (PMD), a source of additional noise.
Another option is the use of photonic bandgap fibers, where the signal propagates in a central air core. In this case, most of the signal energy resides in air, and hence undergoes negligible amounts of SPM-based pulse broadening. However, photonic bandgap fibers are difficult to manufacture in comparison to doped fibers, and hence are not a cost-effective solution. Geometric regularity problems are severely exacerbated, leading to the possibility of high PMD and associated problems. They also suffer from the inability to splice a mode-shaping element at the fiber output, because the splice causes the photonic bandgap effect to disappear, and will yield large losses.
Another proposal is to use pulse shaping schemes to combat the nonlinear broadening in standard fibers. The pulse is temporally chirped, and spectrally narrowed before launching into the delivery fiber. While this produces short pulses, the power levels are low and not desirable for two-photon applications.
New approaches that can minimize the deleterious effects of nonlinear SPM, while maintaining the advantages of a standard fiber, such as low propagation and bend losses, low PMD, ability to splice to GRINs and other lenses, and high manufacturing yield and control, would represent a significant advance in the technology.